The present invention relates generally to electrical test probes for making contact between electrical components and testing instruments and, more specifically to a self-calibrating differential electrical testing probe.
An electrical test probe 18 generally consists of a probing head 20, a cable 22, and a scope interface (hereinafter, “connector 24”). The probing head 20 may have an integral or replaceable probing tip 26 or other test probe input that is suitable for making an electronic contact with electrical components. The probing head 20 is attached to a first end of the cable 22 and the connector 24 is attached to a second, opposite end of the cable 22.
Test probes are often used to provide an electrical connection between electrical components 28 (e.g. a component lead, wire, circuit board trace, electrical contact, or other component such as those located on a circuit board) and testing instruments 30 (e.g. oscilloscopes, voltmeters, and signal measuring instruments). The electrical connection is made by attaching the connector 24 at the second end of the cable 22 to a testing instrument 30 and using the probing head 20 (and usually an integral or replaceable probing tip 26) at the first end to touch, grip, permanently attach, semi-permanently attach (e.g. soldering), plug into, or otherwise make contact with an electrical component 28. It should be noted that the connector 24 may be integral with the testing instrument 30. As an electrical component 28 is probed with a test probe, the signal flowing through the electrical component 28 registers on the testing instrument 30.
In order to provide correct measurements, testing instruments 30 should be calibrated as accurately as possible. The calibration that is generally used in instruments that measure amplitude improves accuracy by compensating for one or both of the gain and offset errors. Modern testing instruments 30 can calibrate the entire signal path including the testing instrument 30 and the test probe. The process of calibrating the entire signal path can be described in two phases: calibration of the testing instrument 30 and calibration of the test probe. As shown in FIG. 1, calibration of the testing instrument 30 can be done automatically in that it does not require user intervention. Exemplary testing instruments capable of self-calibration include, for example, the LeCroy WavePro series of digital oscilloscopes. As shown in FIG. 2, to calibrate the test probe, the user must remove the probing head 20 from the electrical component 28 (if the probing head 20 is attached to the electrical component 28), attach the probing head 20 to a probe compensation port on the testing instrument 30, and run the calibration routine. When calibration is complete, the user may then reestablish a connection by replacing the probing head 20 on the electrical component 28. Calibration of the test probe, therefore, requires significant user intervention in which the user must perform the manual step of attaching the probing head 20 to a probe compensation port before the actual calibration of the test probe is performed by the testing instrument 30. To find the calibration of the entire signal path, the information from the calibration of the testing instrument 30 is combined with the information from the calibration of the test probe.
Removing and replacing a probing head 20 from an electrical component 28 is problematic because it is often difficult and/or time consuming to accurately place, position, and hold the probing head 20 on the electrical component 28. Further, removing and replacing a probing head 20 from an electrical component 28 may cause damage to the electrical component 28. For example, for sensitive components where the contact area is a soft metal, each time a connection is made the metal may be braded or scraped off. Future connections, therefore, would not be as accurate because the contact would not be as complete. Removing the probing head 20 from the electrical component 28, therefore, is undesirable.
Calibration may be done each time a test probe is connected to a testing instrument 30, each time a testing instrument 30 is turned on, at regularly timed intervals, when the instrument configuration changes, upon a change in temperature, and/or at the user's request.
As mentioned, calibration may be done each time a test probe is connected to a testing instrument 30. To replace a test probe, a user disconnects the current test probe from the testing instrument 30. Then the user connects a new test probe to a testing instrument 30. Test probes are generally replaced when the user needs a new test probe with characteristics that are appropriate for a particular use. The user may also replace a test probe if the current test probe is malfunctioning. Changing a test probe may change parameters such as attenuation, capacitance, frequency response, component tolerances, manufacturing variations, and other calibration variables. Calibrating particular combinations of the testing instrument 30, the testing instrument channel, and the test probe helps to obtain and maintain high measurement accuracy. If in the process of connecting the test probe to the testing instrument 30 and/or the electrical component, the test probe breaks or other calibration variables change, the known testing instruments 30 do not have any provision to deal with this.
Calibration may also be performed each time a testing instrument 30 is activated (e.g. turned on). Testing instruments 30 are generally turned off when they are not in use and must be turned on again when they are to be used. This may happen several times a day and each time the user must remove the probing head 20 from the electrical component 28 and then replace the probing head 20 back on the electrical component 28 when the calibration is complete.
Calibration that is performed at regularly timed intervals (e.g. every two hours), when the frame configuration changes, and/or upon a change in temperature (e.g. when the ambient temperature changes more than 10 degrees C. from the temperature at the time of the last full calibration), may also occur several times a day. At each instance of calibration, the user must remove the probing head 20 from the electrical component 28 and then replace the probing head 20 back on the electrical component 28 when the calibration is complete.
To optimize the accuracy of a particular measurement, a user may also request calibration. A user requested calibration again requiring the user to remove the probing head 20 from the electrical component 28 and then replace the probing head 20 back on the electrical component 28 when the calibration is complete.
If calibration is performed several times a day, the added time and trouble of removing the probing head 20 from the electrical component 28 and then replacing the probing head 20 back on the electrical component 28 may cause significant problems including added time, possible misconnections, poor connections with the contact, and other undesirable consequences of removal and replacement.